Charge distributions in KTa1₋xNbxO3 optical beam deflectors formed by voltage application.

Controlling the space charge distributions in a crystal is indispensable for controlling a KTa1₋xNbxO3(KTN) optical beam deflector. The space charge is built up by applying a voltage and injecting electrons into the KTN crystal. Although a homogeneous distribution is preferable, we observed experimentally that the injected electrons concentrated in the vicinity of the cathode and for some samples the concentration was much lower around the anode. We investigated the electron dynamics theoretically and found that such inhomogeneity was caused by a freezing effect where the motion was very slow considering the duration of the practical voltage application. The depth of the space charge spread or the electron penetration depth from the cathode was proportional to the applied voltage and the permittivity, and inversely proportional to the density of traps or localized states that bind electrons. We believe that the trap density was too large for the samples with inhomogeneous charge distributions.

Trapped charge density analysis of KTN crystal by beam path measurement.

Because the function of a single crystal of potassium tantalate niobate (KTa(1-x)Nb(x)O(3), KTN) is largely decided by the trapped charge density inside it, it is essential to determine its value. We quantitatively estimate the charge density using two optical analysis methods, namely by investigating KTN's deflection angle when it is used as a deflector and by investigating KTN's focal length when it is used as a graded-index (GRIN) lens. A strobe technique is introduced with which to perform the measurement. The charge density values under different temperature conditions are shown. These results suggest that the charge density can be determined with both methods, and is constant in a specific temperature range. The charge density value is around 80 C/m(3) in our setup.

Polarization independent varifocal lens using KTN crystals.

We fabricated a polarization-independent varifocal lens using KTa(1-x)Nb(x)O3 (KTN) crystals. The polarization dependence of the KTN crystal is effectively compensated for by combining a pair of KTN lenses and a half-wave plate. This compensation is achieved by a total electro-optic effect, which consists of the Kerr effect and the elasto-optic effects via the electrostrictive and elastic strains in the KTN crystal.

Fast response varifocal lenses using KTa(1-x)Nb(x)O3 crystals and a simulation method with electrostrictive calculations.

We fabricated cylindrical varifocal lenses with fast responses by using the strong Kerr effect of KTa(1-x)Nb(x)O(3) (KTN) single crystals. We observed focus shifts of up to 87 mm with the assistance of a 250 mm focal length lens, which corresponds to a focus shift from infinity to 720 mm by the KTN lens itself. The response time was as fast as 1 µs. We also present a simulation method for calculating refractive index distributions in KTN single crystals, which is essential when designing the lens. The method is characterized by the strain contribution, which has not conventionally been typical of electro-optic simulations. We used this method to explain the refractive index modulations that are characteristic of the varifocal lenses.